Bone fixation device

ABSTRACT

A bone fixation device is provided that includes one or more plates and one or more screws having an acruate outer surface. One or more fastener-receiving apertures with a frustoconical inner surface extend through the plate. The bone fixation device may also include one or more locking arrangements that are configured to secure one or more screws in one or more fastener-receiving apertures.

BACKGROUND

1. Field of the Invention

The invention relates generally to bone fixation devices, and inparticular, bone fixation devices for fusion of the human cervical spinefrom an anterior approach.

2. Background

Cervical plating systems are used to align and maintain adjacentcervical vertebrae in a selected spatial relationship to facilitatefusion of the vertebrae. Generally, cervical plating systems includeplates and screws for aligning and holding vertebrae in a desiredposition relative to one another. In known plating systems, there havebeen problems with loosening and failure of the hardware. In particular,there have been problems with the screws migrating from the plate andpotentially damaging the patient's throat.

Additionally, there have been problems with “creeping substitution” inknown plating systems. In “creeping substitution”, bone at the interfacebetween the graft and vertebrae is removed by natural biologicalprocesses prior to the growth of new bone. Although the plates arecapable of holding the vertebrae in proper alignment, they tend to holdthe vertebrae apart during resorption of the bone, thus forming gaps atthe fusion site. Consequently, fusion may not occur. Such failure isknown as pseudoarthrosis. When such a failure occurs, the hardwareitself may break or become loosened from the spine, and may thereforerequire further surgical procedures to remove the broken components andto attempt fusion.

SUMMARY

The invention provides a bone fixation device that includes one or moreplates and one or more fasteners. The one or more plates each have abone-contacting surface and an upper surface opposite thebone-contacting surface. One or more fastener-receiving apertures extendthrough the plate from the upper surface to the bone-contacting surface.According to the invention, one or more fastener-receiving apertureshave a frustoconical inner surface and one or more fasteners have a headwith an arcuate outer surface. In one embodiment, the fastener is ascrew. The bone fixation device may also include one or more lockingarrangements that are configured to secure one or more fasteners in oneor more apertures.

In one embodiment, the locking arrangement includes a bore within theupper surface of the plate, wherein the inner surface of the boreincludes at least one groove. The locking arrangement also includes atleast one locking element having a head configured to secure thefastener within the fastener-receiving aperture and a base having atleast one projection configured to be received within the at least onegroove. In another embodiment, the locking arrangement includes a boreand a locking element, wherein a deformable member is disposed betweenthe locking element and the inner surface of the bore. If desired, thedeformable member can be used in connection with a threaded bore andlocking element to provide a locking arrangement with a predeterminedinitial and final position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a plate of one embodiment of the bonefixation device of the invention.

FIG. 2 is a bottom plan view of a plate of one embodiment of the bonefixation device of the invention.

FIG. 3 is an end elevational view of a plate of one embodiment of thebone fixation device of the invention.

FIG. 4 is a side elevational view of a plate of one embodiment of thebone fixation device of the invention.

FIG. 5 is a cross sectional side elevational view of one embodiment of aplate with apertures having a frustoconical inner surface.

FIG. 6 is a cross sectional side elevational view of an alternativeembodiment of a plate with apertures having a frustoconical innersurface.

FIG. 7 is a cross sectional side elevational view of another embodimentof a plate with apertures having a frustoconical inner surface.

FIG. 8 is a top plan view of a plate of another embodiment of the bonefixation device of the invention.

FIG. 9 is a top plan view of a plate of another embodiment of the bonefixation device of the invention.

FIG. 10 is a side elevational view of a variable screw.

FIG. 11A is a side elevational view of a fixed screw.

FIG. 11B is a side elevational view of a fixed screw.

FIG. 12 is a top plan view of a plate with multi-lock lockingarrangements.

FIG. 13 is an enlarged view of a locking aperture.

FIG. 14 is a top plan view of a locking element.

FIG. 15 is a side elevational view of a locking element.

FIG. 16 is a side elevational view of an alternative embodiment of alocking element.

FIG. 17 is a side elevational view of an alternative embodiment of alocking element.

FIG. 18 is a side elevational view of an alternative embodiment of alocking element.

FIG. 19 is an enlarged bottom view of a grooved locking aperture with alocking element in a locked position.

FIG. 20 is a longitudinal cross sectional view of a plate with threadedlocking apertures.

FIG. 21 is a longitudinal cross sectional view of a plate with a groovedlocking aperture.

FIG. 22 is a longitudinal cross sectional view of a plate with analternative embodiment of a grooved locking aperture.

FIG. 23 is a schematic of a groove pattern.

FIG. 24 is a schematic of an alternative groove pattern.

FIG. 25 is a cross sectional view of a locking aperture with analternative groove pattern.

FIG. 26 is an enlarged view of an alternative embodiment of a lockingaperture.

DETAILED DESCRIPTION

The invention provides a bone fixation device that includes one or moreplates and one or more fasteners. Although the invention is describedwith respect to a bone fixation device for fusing vertebrae, it is notso limited. The device may be used in the fixation of a wide variety ofbones. If desired, the plate can be used in combination with spinalfusion implants, such as those described in U.S. Pat. No. 5,458,638, thedisclosures of which is incorporated herein in its entirety.

1. Plates

Referring now to FIG. 1, the bone fixation device includes one or moreplates 10. Although the plate 10 shown in FIG. 1 is designed as ananterior cervical plate (i.e., for use in a two level fusion for fusingthree adjacent cervical vertebrae), the plate 10 can be used to fuseother bones, including other vertebrae (e.g., thoracic or lumbar) orother structural bones, such as long bones (e.g., femur or humerus).

The plates 10 each have a bone-contacting surface 111 and an uppersurface 12 opposite the bone-contacting surface (See FIGS. 3-7). One ormore fastener-receiving apertures 13 extend through the plate from theupper surface 12 to the bone-contacting surface 11. In one embodiment,the apertures 13 are configured to receive bone screws (See e.g., FIGS.10 and 11). However, in alternative embodiments the plate may be securedto a bone via a fastener such a nail, spike, tack, staple, wire, rivet,hook, clamp, molley/anchor, expandable fastener, bolt, etc. . . . Ifdesired, the fastener-receiving apertures 13 can include lobes orlateral projections 5, typically at the corners and the center of thesides of plate 10. Generally, the lobes 5 have a rounded outline.Whether or not the fastener-receiving apertures 13 are located withinlobes 5, the apertures 13 are generally located at the corners of theplates in paired sets.

In one embodiment, the plate 10 has a generally elongated form. Forexample, the outline of the plate 10 can be generally rectangular oroval in shape. The elongate form is generally suitable for use in fusingthree or more adjacent vertebrae, or adjacent segments of long bones orother adjacent structural bones. In another embodiment, the plate 10 hasa more truncated form (FIG. 8). The more truncated form may be suitablefor fusing two vertebrae. In yet another embodiment, one or more plates10 can be linked together to form a single device (FIG. 9). In oneembodiment, the plates 10 are linked together in a rigid or fixedconfiguration, for example, using one or more locking tabs 7. As usedherein the term “rigid or fixed” configuration refers to a configurationin which one or more plates 10 are not moveable with respect to oneanother once they are “locked” together. In another embodiment, theplates 10 are linked together in a dynamic configuration. As usedherein, the term “dynamic” configuration refers to a configuration inwhich one or more plates are moveable with respect to one another whenlinked (i.e., rotatable with respect to one another around thelongitudinal axis A-A of the plate and pivotable with respect to oneanother about a transverse axis a-a of the plate). It is contemplatedthat other shapes for the plate 10 also may be employed.

According to the invention, one or more of the fastener-receivingapertures 13 have an inner surface 14 that is at least in partfrustoconical. As used herein, the term “frustoconical” refers to asurface generated by a line that passes through a vertex at a first endand traces a closed curve at a second end. Generally, the surface isbound by two planes that intersect the surface. Although the planes arefrequently parallel, they need not be. The planes can be oblique or evenconverging. Typically, the closed curve that defines the second end is acircle, but other shapes, including but not limited to, ovals orellipses also are suitable. Generally, the frustoconical inner surfacehas a minor aperture 16, proximate the vertex and a major aperture 15,proximate the closed curve. Generally, the diameter (D) of the majoraperture 15 is greater than the diameter (d) of the minor aperture 16.

In one embodiment, shown in FIG. 5, the frustoconical inner surface 14extends from the upper surface 12 of the plate 10 to the bone-contactingsurface 11, such that the upper surface 12 of the plate 10 defines themajor aperture 15 of the frustoconical inner surface 14 and thebone-contacting surface 111 of the plate 10 defines the minor aperture16 of the frustoconical inner surface 14. In an alternative embodiment,the frustoconical inner surface 14 is located between the upper surface12 and the bone-contacting surface 11, but does not intersect one or theother, or both. For example, the major aperture 15 of the frustoconcialinner surface 14 can be defined by the upper surface 12 of the plate 10,but the minor aperture 16 may be disposed at a location between theupper surface 12 and the bone-contacting surface 111 (FIG. 6). In thisembodiment, the frustoconical inner surface 14 is oriented such that theminor aperture 16 is proximate the bone-contacting surface 11 and themajor aperture 15 is proximate the upper surface 12 of the plate 10.Typically, in this embodiment, a channel 17 extends from the minoraperture 16 of the frustoconical inner surface 14 to the bone-contactingsurface 11 of the plate 10. In this embodiment, the opening having thefrustoconical inner surface 14 can be referred to as a “major opening”19. The channel 17 extending from the major opening to thebone-contacting surface 11 can be referred to as the “minor opening” 20.Typically, the minor opening has a cylindrical inner surface 18 with adiameter (d′) substantially the same as (i.e., within about 2% to about10%) the diameter (d) of the minor aperture 16 of the frustoconicalinner surface 14. In yet another embodiment, the frustoconical innersurface 14 may not intersect either the upper surface 12 or thebone-contacting surface 11 of the plate (FIG. 7). For example, theaperture 13 extending between the upper surface 12 and thebone-contacting surface 11 may include three or more segments: a topsegment 18 extending from the upper surface 11; a middle segment havinga frustoconical inner surface 14; and a bottom segment 17 extending fromthe minor aperture 16 of the frustoconical inner surface 14 to thebone-contacting surface 11 of the plate 12. The inner surface of the topsegment 18, extending from the upper surface 12 of the plate toward themiddle segment can be any suitable shape, including but not limited to,cylindrical, frustoconical, and parabolic. Generally, in the embodimentsshown in FIGS. 6 and 7, the opening having the frustoconical innersurface 14 can be referred to as a “major opening” 19 and the openingproximate the bone-contacting surface having a diameter (d′) less thanor equal to the diameter (d) of the minor aperture 16 can be referred toas a “minor opening” 20. Typically, the major aperture 15 has a diameterbetween about 20% to about 80% greater than the diameter of the minoraperture 16, more typically, the major aperture 15 has a diameterbetween about 30% to about 70% greater than the diameter of the minoraperture 16, most typically the major aperture 15 has a diameter betweenabout 40% to about 60% greater than the diameter of the minor aperture16.

As noted above, generally, the plate 10 has one or more pairs offastener-receiving apertures 13. When the device is used to stabilizevertebrae, the number of pairs of fastener-receiving apertures 13generally corresponds to the number of vertebrae to be fused. Forexample, for a two level (three vertebrae) fusion, the plate 10typically has three pairs of fastener-receiving apertures 13 (FIG. 1).Fastener-receiving apertures 13 may be eliminated for a single level(two vertebrae) fusion (FIG. 8), or additional fastener-receivingapertures 13 may be added if additional levels are to be fused (notshown). In one embodiment, the plate 10 is configured as an anteriorcervical plate suitable for stabilizing between 2 to 5 vertebral bodies.Generally, the length of the plate will vary depending on the desiredend use. For example, a lumbar or thoracic plate may be longer than acervical plate. Typically a cervical plate has a length between about 10mm to about 250 mm, more typically between about 15 mm to about 150 mm,most typically between about 20 mm to about 100 mm.

As shown in FIGS. 3 and 4, the plate 10 may be shaped so that thebone-contacting surface 11 has a bi-concave curvature, concave both inthe longitudinal plane (A-A) (corresponding to a length of the plate 10)and in the transverse plane (a-a) (corresponding to a width of the plate10). This embodiment is particularly advantageous when the bone fixationdevice is used to stabilize a vertebral column. The concave curvature inthe longitudinal plane generally conforms to the shape of the anterioraspect of the vertebral column, allowing for the appropriate lordoticcurvature. The concave curvature in the transverse plane can increasethe contact area between the plate and the vertebrae and improvestability of the plate by reducing rocking of the plate 10 relative tothe vertebral bodies, thereby reducing stress on the implant, thevertebral bodies, and surrounding tissue.

The plate 10 can be constructed from any suitable material. Preferably,the plate 10 is constructed from a biocompatible material such asstainless steel or titanium, titanium alloy, polymers and/or resorbablematerials.

If desired, the bone-contacting surface 11 can have a porous, roughened,and/or textured surface and/or may be coated or impregnated with fusionpromoting substances (such as bone morphogenetic proteins). The texturedsurface 11 can help retain the fusion promoting substances. Methods forproducing textured surfaces are known, and include but are not limitedto, rough blasting, etching, plasma spraying, sintering, stamping,coining, and casting. Suitable fusion promoting substances include, butare not limited to, bone morphogenetic proteins, hydroxyapatite, orhydroxyapatite tricalcium phosphate. Additionally, the plate 10 mayinclude at least in part a resorbable material which can be impregnatedwith the bone growth material, so that the bone growth material isreleased as the plate 10 is resorbed by the body.

2. Fasteners

The bone fixation device also includes one or more fasteners 30 havingan arcuate head. A variety of fasteners 30 are suitable and include, butare not limited to, spikes, wires, rivets, hooks, clamps,molley/anchors, expandable fasteners, nails, tacks, staples, etc. In oneembodiment, the fastener is a screw 30 with an arcuate head. Althoughthe description below focuses on the use of screws as fasteners, theconcepts are intended apply to other fasteners as well.

FIGS. 10 and 11 (A and B) provide side elevational views of variable 31and fixed 32 screws, respectively, suitable for use in the presentinvention. Generally, the screw 30 includes a screw head 33 with anarcuate outer surface 34 that extends between an upper surface 37 of thescrew head 33 and a lower surface 36 of the screw head 33. Generally,each screw 30 has a driver engagement member, for example, a recess 44configured to receive the driver. Alternatively, the driver engagementmember can include a projection that extends from the upper surface 37of the screw head, wherein the projection is configured to mate with anopening in the driver.

As used herein, the term “arcuate outer surface” refers to a curvedsurface having a radial arc. The radius of curvature may vary, dependingon the end use of the plate. For example, when used as a cervical plate,the longitudinal axis A-A may have a curvature that generallycorresponds to an arc along the circumference of a circle having aradius between about 15 cm and about 30 cm, more typically between about20 cm and about 25 cm. The transverse axis a-a may have a curvature thatgenerally corresponds to an arc along the circumference of a circlehaving a radius between about 15 mm and about 25 mm, more typicallybetween about 19 mm and about 21 mm.

The screw also includes a shaft 35 that extends from the lower surface36 of the screw head 34. In some embodiments, the shaft 35 has more thanone segment, for example, the shaft 35 may include an upper shaft 40 anda lower shaft 41. Typically, at least the lower shaft 41 is threadedwith a helical thread 38 to enhance engagement between the screw 30 andthe bone to which it is secured. If desired, the upper shaft 40 can bethreaded, in whole or in part. Additionally, the tip 39 of the shaft 35may be fluted by at least one cut out section so as to make the screwself-tapping.

In some embodiments, it may be desirable to taper the diameter of theshaft 35 of the screw 30. Generally, the diameter of the shaft 35increases from the tip 39 of the screw 30 toward upper shaft 40 near thescrew head 33. In one embodiment, the rate of increase in diameter ismay be greater near the screw head 33. The tapered shape is designed toreduce stress risers and increase strength at the screw-plate junction.

Generally, the arcuate lower surface 36 of the screw head 33 ispositioned adjacent the frustoconical inner surface 14 of thefastener-receiving aperture 13 during fixation of a bone with thedevice. If desired, the arcuate lower surface 36 of the screw head 33can contact the frustoconcial inner surface 14 of the fastener-receivingaperture 13 during fixation of a bone with the device. The interfacebetween the arcuate outer surface 34 and the frustoconical inner surface14 is generally a ring of contact.

While not wanting to be limited by theory, it is believed that thefrustoconical surface improves the contact between the plate 10 and thearcuate fastener head. In contrast to known plates that have a sphericalaperture and a spherical fastener head, which may only have two pointsof contact between the aperture and the fastener head, combining afrustoconical surface with a fastener having an arcuate outer surfaceresults in a ring of contact between the fastener head and the surface,thus improving stability and reducing breakage.

Generally, screw head 33 has a diameter (D′) that is less than thediameter (D) of the major aperture 15 of the frustoconical inner surface14 of the fastener-receiving aperture 13, but greater than the diameter(d) of the minor aperture 16. The diameter (D′) of the screw head 33prevents the screw 30 from being advanced completely throughfastener-receiving apertures 13 of the plate 10.

The screws 30 can be constructed from any suitable material. Preferably,the screw 30 is constructed from a biocompatible material such asstainless steel, or titanium, titanium alloy, polymers and/or resorbablematerials.

As discussed above, the bone-contacting surface 11 of the plate 10 isbi-concave. If one or more screws 30 are secured perpendicular to theradius of curvature A-A of the plate 10, the longitudinal axis A′-A′ ofthe screws 30 are generally convergent and eventually “meet” at thecenter of the radius. Alternatively, it may be desirable to secure oneor more screws 30 at an angle with respect to the radius of curvatureA-A of the plate 10. The bone fixation device of the invention includesfixed and/or variable screws to allow the surgeon to optimize the screwposition.

A. Fixed Screw

In some cases it may be desirable to use a fixed screw 32 in connectionwith the bone fixation device. Two embodiments of fixed screws are shownin FIGS. 1A and 11B. Other embodiments also are envisioned. Generally, afixed screw 32 has little to no angular motion once it is secured withinthe fastener-receiving aperture 13. Typically, the fixed screw 32 isused in combination with a plate 10 that has a major opening 19 with afrustoconical inner surface 14 and a minor opening 20 proximate thebone-contacting surface 11 of the plate 10.

Generally, the fixed screw 32 has an upper shaft 40 that has a diameterthat matches the diameter (d) of the minor opening 20. As used herein,the term “matches” means that the upper shaft 40 is receivable withinthe minor opening 20, but the diameter of the upper shaft 40 is no morethan about 10% less than the diameter (d) of the minor opening 20,typically no more than about 5% less, most typically no more than about2% less. In one embodiment, the upper shaft 40 of the fixed screw 32includes a major component 42 proximate the lower surface 36 of thescrew head 33 and a minor component 43 proximate the lower shaft. Inthis embodiment, the major component 42 has a diameter that matches thediameter of the minor opening 20 of the fastener-receiving aperture 13and the minor component 43 has a diameter that is less than (i.e., atleast about 20% less, more typically at least about 10% less, mosttypically at least about 5% less) the diameter of the minor opening 20.

B. Variable Screw

In some cases it may be desirable to use a variable screw 31 inconnection with the bone fixation device. One embodiment of a variablescrew is shown in FIG. 10. Other embodiments also are envisioned. Thevariable screw 31 can be used in connection with a plate 10 that hasfastener-receiving apertures 13 with or without major 19 and minor 20openings. Generally, a variable screw 31 has an upper shaft 40 with asmaller diameter than that of a fixed screw 32 (i.e., when designed foruse with the same plate), such that the screw 31 can be positioned at anangle relative to the longitudinal axis L-L of the fastener-receivingaperture 13. When a variable screw 31 is used in connection with theplate 10 of the invention, the plate 10 can self-adjust (e.g., rotatedown) as the bone that is being stabilized undergoes creepingsubstitution or other settling, such that the load on the device isshared between the screws, the plate and the vertebrae. Alternatively,the surgeon may want to angle one or more screws 31 at the time ofimplantation for other reasons. For example, when stabilizing cervicalvertebrae, it may be desirable to angle the top-most screw to reduce thelikelihood that the screw will enter the disc space.

Generally, the variable screw 31 includes a screw head 33 having anarcuate outer surface 34 with a diameter that is greater than thediameter of the minor opening 20 and an upper shaft 40 with a diameterthat is less than the diameter of the minor opening 20. Generally, theupper shaft 40 of the variable screw has a diameter that is betweenabout 10% and about 50% less than the diameter of the minor opening 20,more typically between about 20% and about 40% less, most typicallybetween about 25% and about 35% less.

3. Locking Arrangements

In one embodiment, the bone fixation device includes one or more lockingarrangements 50. See, for example, FIG. 1. In some embodiments, theplate 10 includes one locking arrangement 50 for each fastener 30 (a“single-lock” arrangement). In other embodiments, the plate 10 includesone locking arrangement 50 for multiple fasteners (a “multi-lock”arrangement). For example, the plate 10 may include one lockingarrangement for each pair of fastener-receiving apertures 13. Thus, aplate 10 for a one level fusion (two pairs of fastener-receivingapertures 13) might have two locking arrangement 50 (FIG. 8), whileplates for fusing more than two levels (three vertebrae) could haveadditional locking arrangements 50 corresponding to additional pairedbone screw holes (FIG. 1).

FIG. 12 is a top plan view of a plate 10 with multi-lock lockingarrangements 50 for each pair of fastener-receiving apertures 13. Manysuitable locking arrangements 50 are possible. Generally, the lockingarrangement 50 includes a locking element 51 (FIGS. 14-18) housed in alocking aperture 52 (FIG. 13) in the upper surface 12 of the plate 10.In one embodiment, the locking aperture 52 extends from the uppersurface 12 of the plate 10 to the bone-contacting surface 11. In anotherembodiment, the locking aperture 51 extends from the upper surface 12and terminates at a location between the upper surface 12 and thebone-contacting surface 11 of the plate. In yet another embodiment, thelocking aperture 52 includes a first countersink 66 in the upper surface12 of the plate 10 (FIGS. 20 & 21). In this embodiment, the lockingcover 53 of the locking element 51 is typically configured to bereceived within the first countersink 66. In other embodiments, thelocking element 51 is not recessed within the locking aperture 52 (i.e.,the locking element may have a raised profile with respect to the uppersurface 12 of the plate 10).

The locking element 51 can be removable or non-removable, and may or maynot have a locked and unlocked position. In one embodiment, the lockingelement 51 is a rotable element such as a rotatable cam that has anunlocked position that permits insertion of a fastener 30 into thefastener-receiving aperture 13 and a locked position in which a lockingcover 53 blocks the fastener from “backing out” of thefastener-receiving aperture 13. Alternatively, the locking element 51 isa removable element, such as a screw, which can be positioned within thelocking aperture 52 after placement of the screws 30 in thefastener-receiving apertures 13. In this embodiment, the locking cover53 may or may not be cammed. In yet another embodiment, the lockingelement 51 engages the locking aperture 52 via a press fit or frictionfit.

As used herein, the term “cammed” refers to a locking element 51 havinga locking cover 53 that includes one or more cutout portions 70configured to allow the fastener 30 to pass the locking cover 53 andrest within the major opening 19 (FIG. 14). Typically, the locking cover53 of the locking element 51 includes a driver engagement member, suchas a noncircular recess 58, which is configured to engage an appropriatedriver tool.

In one embodiment, the locking element 51 is constructed as a removablescrew having a locking cover 53 and a threaded shaft 57 (FIG. 15),wherein the threaded shaft 57 is configured to mate with an innersurface 54 (FIGS. 13 and 20) of the locking aperture 52 that includesthreads 55.

In an alternative embodiment, the locking element 51 is constructed as arivet (FIG. 16) that includes a locking cover 53, a shaft 57, and aflange 59. The rivet is configured to mount to the locking aperture 52.The rivet can be a separate, removable component from the plate, or itcan be non-removable, formed as part of the plate 10 during themanufacturing process.

The locking element 51 can be a “cap” 71 that is merely a locking cover53 (FIG. 17) with a threaded exterior surface 60, wherein the threads onthe locking cover 53 are configured to mate with an inner surface 54 ofthe locking aperture 52, which includes one or more threads 55(typically helically wound threads).

A. Projection/Groove Locking Arrangement

In one embodiment, the locking arrangement 50 includes at least onelocking aperture 52 within the upper surface 12 of the plate 10, whereinthe locking aperture 52 has an inner surface 54 including at least onegroove 61 (FIGS. 19 to 25). The locking arrangement 50 also includes atleast one locking element 51 that includes a locking cover 53 configuredto secure the fastener 30 within the fastener-receiving aperture 13 anda shaft 57 that includes at least one projection 62 configured to bereceived within the at least one groove 61 (FIG. 18). To secure thelocking element 51 within the locking aperture 52, at least oneprojection 62 of the locking element 51 is aligned with at least onegroove of the locking aperture 52. Generally, one or more projections 62are configured to be received within one or more grooves 61. In oneembodiment, the projections 62 are configured to be received within thegrooves when the locking arrangement 50 is in a locked position. Inalternative embodiments, the projections 62 may be configured to bereceived within the grooves 61 when in an unlocked position. Someembodiments are described below. In one embodiment, at least oneprojection 62 is compressed radially inwards when the locking element 51is in an unlocked position and expands radially outward into at leastone groove 61 when in the locking element 51 is in a locked position.The locking aperture 52 may or may not extend from the upper surface 12of the plate 10 all the way to the bone-contacting surface 11.

The locking element 51 can include one projection 62 configured to bereceived within at least one groove 61 or a plurality of projections 62,typically between two and four, preferably three or four projectionsconfigured to be received within a plurality of grooves. Typically, thenumber of grooves 61 is the same as, or greater, than the number ofprojections 62.

In one embodiment, one or more projections 62 of the locking element 51extend along the longitudinal axis (a′-a′) of the locking element 51. Inone embodiment, one or more projections 62 include distinct “legs” 63that extend from the locking cover 53. The term “legs” 63 is meant todistinguish this embodiment from others in which one or more projections62 protrude, similar to a ridge or bump, from the shaft of the lockingelement 51. In a preferred embodiment, the locking element 51 includesthree legs 63 that extend from the base of the locking cover 53 (FIG.18). The locking aperture 52 may include grooves 61 that extend in adirection configured to receive the projections 62 when the lockingelement 51 is in a locked position (FIGS. 19 and 21). The grooves 61 mayor may not extend all the way from the upper surface 12 of the plate 10to the bone-contacting surface 11.

Other embodiments for a “snap-fit” locking arrangement are envisioned.For example, the locking element 51 may include a locking cover 53 and ashaft 57 that has one or more projections 62 that are orientedtransverse to the longitudinal axis (a′-a′) of the locking element 51.In this embodiment, the locking aperture 52 may have one or more grooves61 that are configured to receive the projections 62 of the lockingelement 51 when the locking element 51 is in a locked position. Forexample, one or more grooves may extend “horizontally” within thelocking aperture 52 (FIG. 25). As the locking element 51 is advancewithin the locking aperture 52, one or more projections 62 arecompressed radially inward. Once the locking element 51 is advanced suchthat the projections 62 align with one or more grooves 62, one or moreprojections 62 expand radially outward, into the groove 62. Thehorizontal groove 62 has an upper 72 and a lower 73 surface. Typicallythe upper surface 72 is oriented in a direction substantially transverseor perpendicular to the longitudinal axis L-L of the locking aperture 52to secure the projection 62 within the groove 61. The lower surface 73may be ramped or angled with respect to the longitudinal axis L-L of theaperture 52, to facilitate advancement of the locking element 51 withinthe aperture 52.

Other groove configurations are shown in FIGS. 23 and 24. A lockingelement 51 having one or more projections 62 configured to fit within agroove 61 having configuration shown in FIG. 23 can be used. In thisembodiment, the one or more projections 62 “track” the groove 61 as thelocking element 51 is advanced within the locking aperture (i.e., theprojections 62 are present within the groove when the locking element isin both the locked and unlocked position). As the locking element 51 isadvanced into the fastener-receiving aperture 13, the projections 62“track” the grooves 61, and the locking element 51 is rotated generallyin the direction of arrow “B”. When the locking element 51 is positionedat a desired location, the locking element 51 can be rotated in thedirection of arrow “A” until the projection 62 rests within the lockingpocket 70. If it is desirable to advance the locking element 51 furtherwithin the locking aperture 52, the locking element 51 can be rotated inthe direction of arrow “B”.

Yet another groove configuration is shown in FIG. 24. In thisconfiguration, one or more projections 62 on the shaft 57 of the lockingelement 51 are configured to fit within a groove 61 having theconfiguration shown. The locking element 51 is rotated in the directionof arrow “C” to rotatably advance the locking element 51 into thelocking aperture 52.

Still other groove configurations are envisioned and fall within thescope of the invention.

In one embodiment, the locking element 51 may also include one or moreanchors 64 configured to secure the locking element 51 within thelocking aperture 52. In one embodiment, the anchor 64 is a flange thatextends radially outward from shaft 57 of the locking element 51, mosttypically from the tip 65 of the shaft 57. In one embodiment, the anchor64 is configured to contact the bone-contacting surface 11 of the plate10. In an alternative embodiment, the locking aperture 52 includes asecond countersink 67 in the bone-contacting surface 11 of the plate 10.The anchor 64 of the locking element 51 is configured to be receivedwithin the second countersink 67.

B. Deformable Member

In another embodiment, the locking arrangement 51 includes one or morelocking elements 51, one or more locking apertures 52, and a deformablemember 68 disposed between the surface 54 of the locking aperture 52 andthe surface 69 of the shaft 57 of the locking element 51. According tothis embodiment, the deformable member 68 deforms when the lockingelement 51 is secured within the locking aperture 52. The deformation ofthe deformable member 68 helps secure the locking element 51 within thelocking aperture 52.

A variety of locking elements 51 can be used in connection with thedeformable member, many of which are described above. In one embodiment,the locking element 51 is rotatably mounted within the locking aperture52. In an alternative embodiment, the locking element 51 is securedwithin the locking aperture 52 by frictional engagement or a snapengagement.

The deformable member 68 can be any suitable size and shape. In oneembodiment, the deformable member 68 is in the shape of a cylinder andhas a length about its longitudinal axis that is less than or equal tothe length of a longitudinal axis of locking aperture 52. Typically, thedeformable member 68 is smaller than the aperture 52. More typically,the deformable member is at least about 50% smaller than the aperture.The deformable member 68 can be mounted to the inner surface 54 of thelocking aperture or to the shaft 57 of the locking element 51.Preferably, the deformable member 68 is constructed from a biocompatibleplastic material. In one embodiment, the deformable member 68 isconstructed from ultra high molecular weight polyethylene (UHMWPE).

C. Timing

In another embodiment, the locking arrangement 50 includes one or morethreaded rotatable locking elements 51 having a predetermined lockedposition and/or a predetermined unlocked position and one or morelocking apertures 52 with an inner surface 54 that includes threads 55.The threads of the locking element 51 are configured to mate with thethreads 55 of the locking aperture 52 such that the base has apredetermined initial and final position. Various timing arrangementsare known.

D. Timing Combined with Deformable Member

In another embodiment, the locking arrangement 51 includes a timingmechanism, such as the one described above, in combination with adeformable member 68. In this embodiment, the timing mechanismdetermines at least a final position of the locking element 51, and mayalso determine an initial position, if desired. The deformable member 68prevents the locking arrangement 51 from “unwinding” once the finalposition is obtained.

According to this embodiment, a deformable member 68 is disposed betweenthe inner surface 54 of the locking aperture 52 and the surface 69 ofthe shaft 57 of the locking screw 56. The threads of the locking element51 are configured to mate with the threads 55 of the locking aperture 52such that the base has a predetermined initial and final position. Asthe locking element 51 is rotated, the deformable member 68 becomesdeformed, making it more difficult to “unwind” the locking member.

From the foregoing detailed description and examples, it will be evidentthat modifications and variations can be made in the devices and methodsof the invention without departing from the spirit or scope of theinvention. Therefore, it is intended that all modifications andverifications not departing from the spirit of the invention come withinthe scope of the claims and their equivalents.

1-38. (canceled)
 39. A bone fixation device, comprising: one or moreplates, each comprising: a bone-contacting surface; an upper surfaceopposite the bone-contacting surface; and one or more fastener-receivingapertures extending through the plate from the upper surface to thebone-contacting surface, wherein at least one fastener-receivingaperture comprises: a major opening proximate the upper surface, whereinthe major opening comprises a frustoconical inner surface; and a minoropening proximate the bone-contacting surface, wherein the minor openingcomprises a cylindrical inner surface; and one or more screws includinga head having an arcuate outer surface with a diamter greater than adiameter of the minor opening, wherein the arcuate outer surface of thescrew head is positioned adjacent the frustoconical inner surface of thefastener-receiving aperture during fixation of a bone with the device,the further including an upper shaft having a diameter that matches thediameter of the minor opening. 40-42. (canceled)
 43. A bone fixationdevice, comprising: one or more plates, each comprising: abone-contacting surface; an upper surface opposite the bone-contactingsurface; and a locking arrangement, comprising: at least one lockingaperture within the upper surface of the plate, the locking aperturehaving an inner surface comprising at least one groove; and at least onelocking element, comprising: a locking cover configured to secure thescrew within the fastener-receiving aperture; and a base comprising atleast one projection configured to be received within the at least onegroove.
 44. The bone fixation device according to claim 43, wherein oneor more projections are configured to be received within one or moregrooves when the locking element is in a locked position.
 45. The bonefixation device according to claim 43, wherein the shaft of the lockingelement comprises a plurality of projections.
 46. The bone fixationdevice according to claim 45, wherein the locking aperture comprises aplurality of grooves.
 47. The bone fixation device according to claim43, wherein the locking aperture further comprises a first countersinkproximate the upper surface of the plate.
 48. The bone fixation deviceaccording to claim 47, wherein the locking element includes a lockingcover that is configured to be received within the first countersink.49. The bone fixation device according to claim 47, wherein the lockingaperture comprises a second countersink proximate the bone-contactingsurface of the plate.
 50. The bone fixation device according to claim49, wherein the locking element comprises an anchor configured to bereceived within the second countersink.
 51. The bone fixation deviceaccording to claim 43, wherein one or more projections are configured tobe received within at least one groove wherein when the locking elementis in the locked position.
 52. The bone fixation device according toclaim 43, wherein the at least one projection is compressed radiallyinwards when the locking element is in an unlocked position and expandsradially outward in one or more grooves when in a locked position.
 53. Abone fixation device, comprising: one or more plates, each comprising: abone-contacting surface; an upper surface opposite the bone-contactingsurface; and a locking arrangement, comprising: at least one lockingaperture within the upper surface of the plate, the bore having an innersurface comprising at least one groove; and at least one rotatablelocking element having a locked and unlocked position, the lockingelement comprising: a locking cover configured to secure a screw withinthe fastener-receiving aperture when in the locked position; a shafthaving a tip comprising at least one projection configured to bereceived within the at least one groove; and an anchor proximate the tipof the shaft, wherein the anchor is configured to be received within arecess of the groove when in the locked position.
 54. A bone fixationdevice, comprising: one or more plates, each comprising: abone-contacting surface; and an upper surface opposite thebone-contacting surface; and at least one locking arrangement rotatablymounted to the one or more plates, comprising: at least one lockingaperture within the upper surface of the plate, the locking aperturehaving an inner surface; and at least one locking element, comprising: alocking cover configured to secure the one or more screws in the one ormore fastener-receiving apertures; a shaft rotatably mounted within thelocking aperture; and a deformable member disposed between the shaft ofthe locking element and the inner surface of the locking aperture. 55.The bone fixation device according to claim 54, wherein the lockingarrangement has a locked and an unlocked position
 56. The bone fixationdevice according to claim 54, wherein the locking aperture extendsbetween the bone-contacting surface and the upper surface of the plate.57. The bone fixation device according to claim 54, wherein thedeformable member comprises a deformable cylinder
 58. The bone fixationdevice according to claim 54, wherein the deformable member is mountedto the inner surface of the locking aperture.
 59. The bone fixationdevice according to claim 54, wherein the deformable member is mountedto a shaft of the locking element.
 60. The bone fixation deviceaccording to claim 54, wherein the deformable member is constructed froma biocompatible plastic material.
 61. The bone fixation device accordingto claim 54, wherein the deformable member is constructed from ultrahigh molecular weight polyethylene (UHMWPE).
 62. A bone fixation device,comprising: one or more plates, each comprising: a bone-contactingsurface; and an upper surface opposite the bone-contacting surface; andat least one locking arrangement rotatably mounted to the one or moreplates having a predetermined locked position, comprising: at least onelocking aperture within the upper surface of the plate, the lockingaperture having a threaded inner surface; and at least one lockingelement, comprising: a locking cover configured to contact the uppersurface of the plate; and a threaded shaft rotatably mounted within thelocking aperture, wherein the threads of the base are configured to matewith the threads of the locking aperture such that the locking elementhas a predetermined initial and final position; a deformable memberdisposed between the shaft of the locking element and the inner surfaceof the locking aperture.
 63. The bone fixation device according to claim62, wherein the locking arrangement has a locked and an unlockedposition
 64. The bone fixation device according to claim 62, wherein thelocking aperture extends between the bone-contacting surface and theupper surface of the plate.
 65. The bone fixation device according toclaim 62, wherein the deformable member comprises a deformable cylinder66. The bone fixation device according to claim 62, wherein thedeformable member is mounted to the inner surface of the lockingaperture.
 67. The bone fixation device according to claim 62, whereinthe deformable member is mounted to a shaft of the locking element. 68.The bone fixation device according to claim 62, wherein the deformablemember is constructed from a biocompatible plastic material.
 69. Thebone fixation device according to claim 62, wherein the deformablemember is constructed from ultra high molecular weight polyethylene(UHMWPE).